This invention relates to an antenna assembly comprising a whip antenna and a helical antenna mounted on a top end of the whip antenna.
An antenna assembly of the type described has been used in a radio communication device such as a portable communication terminal set, especially a mobile telephone terminal set. The antenna assembly is usually extendably and retractably mounted to a housing (or a casing) of the terminal set as disclosed in JP-A-3 245603 (Reference I).
In Reference I, the terminal set has a housing or enclosure enclosing transmitting and receiving electrical circuitry. The antenna assembly comprises the whip antenna (or an antenna rod) and a holder (or a support) attached to the housing for slidably holding (or supporting) the whip antenna. The holder (or the support) is made of a conductor and is connected to the electrical circuitry. The whip antenna is provided with a stopper (or a conductive ring) fixedly mounted on a lower or an inner end. When the whip antenna is in an extended position, the stopper is brought into contact with the holder (or the support) so that the whip antenna is connected to the electrical circuitry through the stopper and the holder (or the support). The whip antenna comprises a conductive rod covered with a dielectric sleeve or tube. The whip antenna or the conductive rod has an electrical length of a quarter wavelength of a predetermined frequency.
The helical antenna (or an antenna coil) is enclosed in a dielectric cap and is carried on a top end of the whip antenna. The dielectric cap is provided with a conductive sleeve at a lower end electrically connected to the helical antenna. The conductive sleeve is fitted onto the top end of the whip antenna and fixed thereto by caulking or deforming the conducive sleeve together with the dielectric sleeve of the whip antenna. The helical antenna is connected to the conductive rod of the whip antenna and has also an electrical length of a quarter wavelength of the predetermined frequency. Therefore, the antenna assembly has a half wavelength of the predetermined frequency.
When the antenna assembly is in a retracted position where the whip antenna is retracted in the housing, the helical antenna is connected to the electrical circuitry through the conductive sleeve and the holder (or the support). Thus, the helical antenna is used for short-range operation of the terminal set. At the retracted position, the whip antenna is in the housing and, therefore, does not serve for receiving the radio signal.
For a long-range operation, the antenna assembly is pulled out by manually handling the cap into the extended position where the stopper is brought into contact with the holder (or the support). Thus, the antenna assembly serves as a half-wavelength antenna. This structure of the antenna assembly will be referred to as a "non-separate type" because the whip antenna is not electrically separated from the helical antenna.
An assembly of the helical antenna and the dielectric cap with the conductive sleeve will be referred to as an antenna top.
Use is made of a special support of a coaxial structure as the holder (or the support) in order to insure that the whip antenna is disabled when the antenna assembly is in the retracted position. This is disclosed in GB 2,257,836 A (Reference II) and JP-A-5 243829 corresponding thereto.
In U.S. Pat. No. 5,204,687 (Reference III) and JP-B-2646505 (Reference IV), another structure of the antenna assembly is disclosed in which the conductor rod of the whip antenna is not electrically connected to the helical antenna but is insulated from the helical antenna. In the structure, the whip antenna is reliably disabled in the retracted position without use of the special support of the coaxial structure. The whip antenna only serves for receiving the radio signal in the extended position because the helical antenna is no longer connected to the holder (or the support). This structure of the antenna assembly will be referred to as a "separate type" because the whip antenna is electrically separated from the helical antenna.
In detail, Reference IV discloses a dielectric joint member of a generally rod shape which is secured at one end thereof to the top end of the conductor rod of the whip antenna. The dielectric joint member is partially covered with the conductive sleeve and is fitted at the other end portion with a coil bobbin. A helical coil or the helical antenna is wound on the coil bobbin and is connected to the conductive sleeve. The dielectric cap covers the coil bobbin, the helical coil, and the top end portion of the conductive sleeve together by, for example, the plastic molding to form the antenna top.
In the above, the conductive sleeve and the top end of the conductive rod of the whip antenna are fixed to the dielectric joint member by the insulation molding of the dielectric joint member when the conductive sleeve and the top end of the conductive rod are inserted into a mold.
In any one of the non-separate type antenna assembly and the separate type antenna assembly, it is essential that each of the holder, the conductive sleeve, and the stopper is electrically conductive. Generally, these conductive portions (namely, the holder, the conductive sleeve, and the stopper) are formed by the use of brass or zinc because of availability, machinability, and platability. As the mobile telephone terminal set becomes smaller in size and lighter in weight, the antenna assembly is also required to be light in size.
However, brass or zinc used as a material of each of the conductive portions of the antenna assembly has a large specific gravity. For example, 70-30 brass has a specific gravity of 8.6 g/cm.sup.2 and zinc has a specific gravity of 7.18 g/cm.sup.2. This makes it difficult to achieve a light weight.